win32mbcs: use str for encoding value This was reported to the TortoiseHg tracker as: https://foss.heptapod.net/mercurial/tortoisehg/thg/-/issues/5980 It doesn't look like we have any tests for this extension, but the explicit type hints are enough to convince pytype that the module level `_encoding` attr is str. The `encode()` and `decode()` methods are too complex to add type hints for them.